The present technology relates to a polypropylene foam having an even foam surface and small cell size. It also relates to the use of multibranched polypropylene for the preparation of foam.
Thermoplastic foams possess a cellular structure generated by the expansion of a blowing agent. The cellular structure provides unique properties that enable the foamed plastics to be used for various industrial applications. Due to their outstanding functional characteristics and low material cost, polypropylene foams have been considered as a substitute for other thermoplastic foams in industrial applications. In particular, they have higher rigidity compared to other polyolefins, offer higher strength than polyethylene and better impact strength than polystyrene. Furthermore, they provide a higher service temperature range and good temperature stability.
However, polypropylene is suffering from some serious drawbacks, limiting its use for the preparation of foams. In particular, many polypropylene materials have low melt strength and/or low melt extensibility.
Polypropylene foams are made in a foam extrusion process wherein a gas-laden melt is suddenly expanded through pressure drop after the extrusion die. Such an expansion induces extensional flow to the polymer melt and sets certain requirements to the extensional rheology of the melt. In particular, high melt strength and/or high melt extensibility are required. If these conditions are not met, bubble film rupture is more likely to occur and the average foam cell size will decrease due to bubble coalescence. Increasing bubble size results in a decrease of impact strength of the foam.
At present, two polypropylene-based systems are used in the industry for the preparation of foams:
1) Linear, bimodal high molecular weight polypropylene obtained either from copolymerisation of propylene with comonomers such as ethylene or from reacting a coupling agent with polypropylene. EP-A-0887379 discloses the preparation of linear bimodal polypropylene in a multi-step process using at least one slurry reactor and at least one gas phase reactor. WO 00/78858 discloses a coupled propylene copolymer prepared by reacting a coupling agent such as polysulfonyl azide with a propylene copolymer.
Due to the high molecular weight, these polymers have a high zero shear viscosity. However, they have reduced output from the extruder because a high pressure is built up in the extrusion line.
2) Y/H-shaped polypropylenes from post-reactor processes such as irradiation or peroxide treatment or from copolymerisation of propylene with dienes in the presence of a metallocene catalyst. WO 2005/044877 discloses a foamed article comprising a propylene/α-ω diene copolymer. EP-A-0879830 discloses Y/H-shaped polypropylene from peroxide treatment.
However, Y/H-shaped polypropylenes are inherently inhomogeneous, contain gels and show a low number of cells, thereby adversely affecting foam surface quality.